The present invention relates generally to transmitting and receiving signals in a mobile radio communication system. In particular the present invention relates to decoding signals in a mobile radio communication system which provides for suspending transmission from a base station of the system during selected time periods.
In time division multiple access (TDMA) radio communication systems, each transmitter in the system is assigned or allocated a time slot for transmitting radio signals. All other transmitters are quiet during this time slot so that the intended receiver can clearly receive the radio signals. If the radio communication system is a two-way mobile system, such as a cellular radio telephone system, a radio carrier frequency in a first frequency band is divided into time slots for communicating from a base station to a plurality of mobile stations, while a radio carrier frequency in a second frequency band is divided into time slots for communicating from different ones of the plurality of mobile stations to the base station. The base station communicates with mobile stations in a fixed geographic area near the base station. Other adjacent base stations serve mobile stations in adjacent areas.
During some of its time slots, the base station transmits control and timing information. This information includes, for example, the identity of a mobile station for which a received call is intended, on a paging channel, and synchronization and timing information on a synchronization or sync channel. A mobile station in the system receives the synchronization channel and synchronizes its timing with base station timing using information in the synchronization channel. Usually, each time slot also includes a known pattern of data to facilitate the mobile station estimating the channel phase and amplitude which is useful for decoding the symbols transmitted in the slot.
It is known to use a synchronization word at the beginning of the current time slot as well as the sync word at the beginning of the next slot and at the end of the current slot to improve the reception and decoding of symbols. Because of a fade or other interference on the channel, the current or first synchronization word may not be reliably received. By storing the received signal samples including the second synchronization word, the stored signal samples may be decoded using either the first or second synchronization word or both. This technique is described in U.S. Pat. No. 5,335,250, issued on Aug. 2, 1994, to Dent, et al., and in U.S. Pat. No. 5,841,816, issued on Nov. 24, 1998, to Dent, et al. Both of these patents are commonly assigned with the present patent application and are incorporated herein by reference. Some mobile stations are presently in use which rely on reception of the second synchronization word for optimal performance.
In some mobile radio systems, it is proposed to suspend or alter transmission of information including synchronization words when a time slot is unallocated. If the time slot is unallocated, no mobile station is presently using that time slot. The could result in the phase, amplitude or direction of transmission by the base station changing in the subsequent time slot with respect to the current time slot which would, without using the invention, degrade the operation of the existing mobile stations. The transmission of information by the base station in an unallocated time slot is however desired to be avoided to reduce interference for receivers in surrounding areas. Elimination of the transmission of the synchronization word and other information during unallocated time slots will reduce or eliminate this co-channel interference improving overall performance in the radio system. This improvement is threatened by the degradation due to removing the mobile station's ability to use the second synchronization word.
For a mobile station which relies on reception of the second synchronization word for improved communication, elimination of the synchronization word or a change in a characteristic such as timing, phase or direction of transmission in unallocated time slots will create a problem if the unallocated time slot follows the time slot allocated to that mobile station. The mobile station may not be able to successfully decode the channel or may spend undue time searching for the second synchronization word. Accordingly, there is a need for method and apparatus for accommodating the elimination of transmission of some slots by base stations in a mobile radio communication system without degrading performance of remaining slots.